The invention relates to coatings for printed circuit boards and the like. It relates, more particularly, to such coatings which have the property of reducing glare and spectral reflectances from the metallic conductors on the surfaces of such boards and from the solder joints affixing electronic components to such boards.
The fabrication and use of printed circuit boards and assemblies is well-known in the electronic arts. Such boards are manufactured by several methods depending on the quantities required and the desired reliability and service life. Discrete components are attached to such boards by manual, semi-automatic, fully automatic wave-soldering, or other soldering equipment.
One of the major costs associated with the manufacture of such circuit boards and assemblies relates to the requirement for close visual inspection of the finished article for the detection of damaged conductors, cold solder joints, breaks, pinholes and other imperfections which could affect the performance of the finished assembly. Such inspection is difficult because of the glare and spectral reflections from the conductors and joints, such reflections commonly appearing as very bright metallic planes and projections. Due to such reflections it is fatiguing to perform such inspections and difficult to assess variations in perceived brightness as to whether they emanate from actual breaks or faults, or represent purely optical phenomena arising from the interaction of the sculptured, mirror-like surfaces and the incident light.
Coating of such circuit boards has been known in the art, and has been undertaken for a plurality of purposes. One such purpose is sealing of the surface against moisture penetration into the substrate and into the interfaces between the substrate and the conductors. One such coating has been disclosed in U.S. Pat. No. 3,034,930 to Foudriat. Another coating is disclosed in U.S. Pat. No. 3,406,285 to Scorgie, et al., wherein the coating is applied prior to a manual soldering operation and contains a fluorescent dye the properties of which change at the proper fusing temperature for the solder used, so that subsequent inspection under ultraviolet illumination can pinpoint joints which have not attained the desired temperature during soldering.
None of the coatings of the prior art are of any actual benefit in performing visual inspections and none eliminate the requirement for such inspection.
It is, therefore, the primary object of the invention to teach the compounding, application and use of a coating which, when applied to a printed circuit board or assembly, will assist in detecting faulty soldering and defective printed conductors.
It is an object of the invention to teach the use of materials in such a coating which permit the performance of rework by common soldering methods after a fault has been detected.
It is another object of the invention to teach light diffusing coatings having the above properties, which may be safely applied by brushing, spraying or other methods, which are non-toxic and non-flammable as applied, and which may be removed with solvents, solutions, and systems normally used for rosin flux.
It is an additional object of the invention to teach the preparation and use of a light diffusing coating which will, by a change in its reflectance, mark the site of rework and, thereby, facilitate subsequent inspection procedures.
It is a further object of the invention to teach the compounding of light diffusing coatings for printed circuit boards and assemblies which make use of commonly available materials and blending processes, are economical to procure, are easy to apply, and have inherent properties conducive to long shelf-life both as raw material and after coating.